Fix lack of media when a user reconnects (#3318)
* Fix lack of media when a user reconnects
This fixes broken media when someone reconnects to the call after
a forced disconnect (when their old call gets replaced immediately
by a new call). We listen for changes in the call feeds and the tearing
down of the feeds for the old call caused us to remove the feed for
the new call.
Also adds the call to the calls map before it'as initialised, such that
it's the active call for the user/device when the feedsChanged event arrives,
otherwise we'll ignore the event.
* Fix tests
